As soft-magnetic materials used for cores for distribution transformers, etc., silicon steel, and ribbons of Fe-based amorphous alloys and Fe-based nanocrystalline alloys are known. Silicon steel is inexpensive and has a high magnetic flux density, but it suffers larger core loss than the Fe-based amorphous alloys. Though the Fe-based amorphous alloy ribbons produced by a rapid quenching method such as a single roll method have lower saturation magnetic flux densities than that of the silicon steel, they have lower core loss because they do not have magnetocrystalline anisotropy for the absence of crystals. Accordingly, they are used for cores for distribution transformers, etc. (for example, see JP 2006-45662 A).
Fe-based nanocrystalline alloy ribbons having nano-sized fine crystal grains, which are formed at high number densities in the alloys by heat-treating the Fe-based amorphous alloys produced by a rapid quenching method such as a single roll method, which may partially have crystal phases, have high saturation magnetic flux densities, as well as higher permeability, lower core loss and lower magnetostriction than those of the Fe-based amorphous alloy ribbons. Accordingly, they are mainly used for cores for choke coils, current sensors, etc. in electronic parts. Known as typical Fe-based nanocrystalline alloys are Fe—Cu—Nb—Si—B alloys, Fe—Zr—B alloys, etc. Fe-based nanocrystalline alloy ribbons having as high saturation magnetic flux density as about 1.8 T, which are suitable for distribution transformer cores, have recently been proposed (see JP 2007-107095 A).
The Fe-based amorphous alloy ribbons are usually produced by a rapid quenching method such as a single roll method, etc. The single roll method is a method for producing an alloy ribbon by ejecting an alloy melt from a nozzle onto a cooling roll made of a high-conductivity alloy, which is rotating at a high speed. The cooling roll is made of Cu alloys with good thermal conduction, such as Cu—Cr alloys, Cu—Ti alloys, Cu—Cr—Zr alloys, Cu—Ni—Si alloys, Cu—Be alloys, etc. To improve productivity, long, wide amorphous alloy ribbons are produced.
The Fe-based amorphous alloys such as Fe—Si—B alloys, etc. used for distribution transformers, etc. have small hysteresis loss because of small magnetic hysteresis. It is known, however, that the eddy current loss (core loss—hysteresis loss) of the Fe-based amorphous alloy in a broad sense is several tens to 100 times as large as the classical eddy current loss determined under the assumption of uniform magnetization. This increased loss is called anomalous eddy current loss or excess loss, which is generated mainly by uneven magnetization change due to large magnetic domain widths of the alloy. To reduce the anomalous eddy current loss, various methods of dividing magnetic domains finer have been attempted.
Known as methods of reducing the anomalous eddy current loss of the Fe-based amorphous alloy ribbon are a method of mechanically scratching a surface of the Fe-based amorphous alloy ribbon (JP 62-49964 B), a laser scribing method of irradiating a surface of the Fe-based amorphous alloy ribbon with laser beams for local melting and solidification by quenching to divide magnetic domains finer, etc. As the laser scribing method, for example, JP 3-32886 B discloses a method of irradiating an amorphous alloy ribbon with pulse laser beams in a transverse direction, thereby melting a surface of the amorphous alloy ribbon locally and instantaneously, and then solidifying it by quenching to form amorphized spots in lines, which divide magnetic domains finer. However, the laser scribing method has low productivity because of a small amount of treatments per a unit area.
JP 61-24208 A discloses a method of controlling the pitches and heights of wave-like undulations in desired ranges at the time of producing an amorphous alloy ribbon having wave-like undulations on a free surface by a single roll method, to divide magnetic domains finer, thereby reducing eddy current loss. This method has higher productivity than the laser scribing method, because the wave-like undulations are formed at the time of production of the amorphous alloy ribbon.
The formation of wave-like undulations on a free surface of an amorphous alloy ribbon produced by a single roll method appears to be due to the vibration of a melt paddle on a cooling roll. However, transverse troughs constituting the wave-like undulations are usually not straight but meandering like waves. Troughs reduce eddy current loss by the division of magnetic domains, but the meandering of transverse troughs increases hysteresis loss. Increased hysteresis loss is serious particularly in wide amorphous alloy ribbons. It is thus desired to provide amorphous alloy ribbons, in which the meandering of transverse troughs constituting the wave-like undulations is as small as possible.
With respect to the suppression of the vibration of a melt paddle, JP 2002-316243 A discloses a method for producing an amorphous alloy ribbon by quenching an alloy melt on a cooling roll, a CO2 gas being blown onto the alloy melt, while the cooling roll is ground. To grind the cooling roll, a brush of brass or stainless steel wires of 0.06 mm in diameter, etc. are used. JP 2002-316243 A describes that if a brush used for grinding were too hard, a ground surface of the cooling roll would have too deep scratches, resulting in cutting of the amorphous alloy ribbons, and little effect of improving surface roughness, and that therefor the brush hardness is preferably equal to or less than the hardness of the cooling roll. However, amorphous alloy ribbons obtained by the method described in JP 2002-316243 A have large core loss despite wave-like undulations on free surfaces.